Abstract

The Selbaie deposits of western Quebec are hosted in calc-alkalic pyroclastic rocks of the Abitibi subprovince. Rocks range upward from subaqueous debris flows through pumiceous tuff to densely welded ignimbrite; they form an intracaldera sequence over 1,200 m thick which records evolution from subaqueous to subaerial deposition. Mineralization occurred within this sequence on the southwest flank of the Brouillan tonalite, a central intrusion and possibly a resurgent dome, and includes regionally extensive and temporally early exhalite, local massive sulfide, and later epithermal veins. These veins were emplaced by open-space filling along synvolcanic structures and make up the bulk of the economic resource.Mineralization and alteration show similarities to both massive sulfide and epithermal deposits. Cu/Zn increases with depth but also increases laterally toward veins and toward the intrusion. Ag/Cu in veins is higher in the upper levels of the system. Whole-rock mass balance calculations indicate enrichment in Si, Fe, Mg, and Mn with proximity to mineralization and depletion in Ca, K, and Na. Na metasomatism, common in host rocks to Archean massive sulfide deposits, is absent in subaerially emplaced tuff at Selbaie and K feldspar is preserved.Selbaie is unusual (1) as an Archean example of a subaerial caldera that evolved from subaqueous to subaerial volcanism, (2) in representing economically important, Archean epithermal mineralization, and (3) in demonstrating the link between subaqueous volcanogenic massive sulfide deposits and subaerial volcanogenic epithermal veins. It is, perhaps, unique in preserving so well all three of these aspects in a single mineralized system. The rarity of similar, mineralized subaerial volcanic settings in the Abitibi subprovince reflects its primitive crustal development, but may also be due to incomplete preservation of subaerial environments.

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